Modern ISDN compatible PABXs typically use a main central control and peripheral interface circuits, for example the PABX type SX2000.TM. sold by Mitel Corporation. Each peripheral interface circuit has a controller which controls various peripherals such as line interface circuits, network interface circuits, etc. The various peripherals communicate with the interface circuits using for example 2B+D, 23B+D or 30B+D line format protocol, the D-channel typically operating at 16 kb/s or 64 kb/s. The peripheral interfaces typically use common channel packet based signalling over the D-channel, such as LAPD (Link Access Protocol for a D-Channel), LAPB (Link Access Protocol, Balanced), MiLAP (Mitel Link Access Protocol), DPNSS (Digital Private Network Signalling System), or DASS 2 (Digital Access Signalling System No. 2). The D-channel data is transmitted using HDLC at 16 kb/s or 64 kb/s to the peripheral control.
Therefore the system requires an HDLC framer at both ends for every ISDN D-channel communication link between the peripheral control and each external interfacing peripheral. With this architecture, the peripheral controller requires significant amounts of memory and computing capability for the processing of all the different link layer protocols as well as translation before transmission to the main central control in the system's specific communication protocol.